Romanian audio specialist Dr. Catalan Grigoras, now director of the National Center for Media Forensics at the University of Colorado, Denver, made an intriguing discovery about a decade ago. The ubiquitous hum of modern society follows a unique pattern that allows many recordings to be validated. Now police in the United Kingdom have begun to use the tactic to verify evidence in important court cases.

I. Industry's Silent Song

Recordings traditionally have been a highly unreliable form of evidence, given that they could easily be cleverly staged or tampered with.

That's where the hum comes in. Electrical sources such as light poles and power outlets emit a near imperceptible hum. While centered around the frequency of the alternating current (50 Hz in the UK), the hum dips and rises by a few thousandths of a hertz over time. The frequency drops when demand outpaces supply, and rises when supply outpaces demand.

Given a long enough window, this pattern of rising and falling frequencies is virtually unique, as Dr. Grigoras found.

[Image Source: UCSC Silicon Valley Extension]

But by using a technique called Electric Network Frequency (ENF) analysis, law enforcement can store the pattern of the hum for a particular grid in a database. The Metropolitan Police lab has been compiling such a database in recent years, as has JP French Associates -- another UK forensics lab.

Comments JP French's Dr. Phillip Harrison to BBC News, "We can extract [the hum from a recording] and compare it with the database - if it is a continuous recording, it will all match up nicely. If we've got some breaks in the recording, if it's been stopped and started, the profiles won't match or there will be a section missing. Or if it has come from two different recordings looking as if it is one, we'll have two different profiles within that one recording."

A trio of gun dealers were sentenced with the help of ENF verification of police evidence.
[Image Souce: PA]

The trio was founded guilty and sentenced to prison for a total of 33 years.

It seems appropriate the novel forensics method has been pioneered in the birthplace of fiction's Sherlock Holmes. But in years ahead, some questions about ENF remain unanswered. For example, while individuals would be unlikely to be able fake the ENF hum, it might be feasible, albeit extremely difficult, for a police force to filter out the hum in a recording and dub in a hum at the time they wish to make the recording appear from, given that they have access to the entire database of recordings.

It might be even possible for a citizen skilled in audio recording to carry out such a feat. Thus the technique may lay to rest questions of cruder tampering, but may still have flaws of its own. For that reason, in time it will probably be used as a piece of a richer evidence puzzle, also composed of other circumstantial clues like cell phone tower records or surveillance footage.

I've done a lot of professional recording while making educational videos. Most of the recording devices used batteries. Many of the remainder used "clean" power passed through a reconditioning power source, designed to eliminate any hum.

On top of that, where is the experimental proof (the empirical evidence) that the hum of a ten or twenty second recording is truly unique?

Totally agree, there does seem to be a huge amount of room for error in this process. Sure, it may actually stand up to scientific scrutiny, but we haven't seen it, and there are guys in jail that may actually have "been locked up and the key thrown away" based on evidence that looks like it is seriously flawed.At the very least the UK police need to explain exactly what equipment was used and how it was used.Personally, I don't buy this "all the grid runs at exactly the same frequency" spiel. It may well be correct, but it seems a huge effort on the part of the UK power authorities to sync the hundreds or thousands of alternators that feed the grid to the exact microsecond (or however accurate was claimed in court). I wouldn't be surprised if there were alternators that ran a fraction fast in peak times while others ran a fraction slow, and as such this pretty well puts this whole evidence in the trash can.

My guess is that if the three guys in jail have good lawyers then they will be getting an appeal on their 17 year convictions pretty soon. Personally, I would hate to find out in 17 years time that they were wrongly convicted.

quote: I would hate to find out in 17 years time that they were wrongly convicted.

What I think has happened is the recording device is where the hum comes from, which doesn't bode well for these guys, because it also means the recorder could easily be using the mains as a clock, and not a crystal controlled clock.I'm not sure what sort of trigger the voice activation would use, but in an extreme case if it was incorrectly set then this could actually give a wrong impression about what conversation took place.